1. Field of the Invention
The present invention relates to a device and a method for conversion from optical double-sideband modulation (DSB) signals to optical single-sideband modulation (SSB) signals by using, particularly, period-one (P1) nonlinear dynamics of semiconductor lasers.
2. Description of the Related Art
Communication networks are generally classified into wireless networks and wireline networks. In the wireless networks, microwaves are used as carriers to deliver data through air to provide communication between mobile electronic devices. In the wireline networks based on optical technologies, optical waves function as carriers to deliver data through optical fibers to provide communication between immobilized electronic devices. These two networks depend on completely different communication approaches and cover completely different communication scopes. Due to the rapid advances of broadband wireless technologies and also due to the various developments of online applications, the capacity demand for data transmission in the wireless networks increases considerably. If the wireless networks are required to manage both the front-end data transmission between users and wireless base stations and the back-end data transmission between the wireless base stations and central offices, currently developed broadband wireless technologies are not capable of meeting the vast capacity demand for data transmission when the wireless networks are simultaneously accessed by a variety of different users or devices.
Since each channel of the wireline networks based on optical technologies provides data transmission capacity of the order of a few Gbits/s to tens of Gbits/s, the optical communication networks are highly suitable to work as backbones for huge back-end data transmission for various network applications. Therefore, radio-over-fiber (RoF) networks which integrate the wireless networks (responsible for front-end data transmission) and the optical wireline networks (responsible for back-end data transmission) have become very attractive for the next generation of communication technology and system. RoF is a promising approach in distributing microwaves over long distances through optical fibers for antenna remoting applications, such as broadband wireless access networks. The RoF networks adopt an architecture where microwaves are generated in the optical domain at central offices and next transmitted to remote wireless base stations through optical fibers. Microwaves are converted to the electrical domain at the wireless base stations using photodetectors, which are next radiated by antennas over small areas.
Since the RoF networks attempt to integrate two different conventional networks, how to superimpose microwaves on optical waves for optical fiber distribution and how to solve physical challenges encountered by the microwave-superimposed optical waves travelling over optical fibers require a number of different functionalities for microwave signal processing. In addition, to reduce construction cost, to expand coverage area, and to increase data capacity, future wireless base stations will be reduced in size, simplified in structure, and developed for high-frequency (such as from 10 GHz to 100 GHz) microwave radiation. Therefore, plenty of microwave signal processing functionalities, which are conventionally carried out using electronic technologies in the wireless base stations, will instead be carried out using optical technologies in the central offices for the RoF networks.
To generate microwaves in the optical domain, direct or external modulation of semiconductor lasers is typically adopted to superimpose microwaves on optical waves. However, either direct or external modulation scheme generates optical double-sideband modulation (DSB) signals, which suffer from significant microwave power fading due to chromatic dispersion over fiber distribution. This microwave power fading effect considerably degrades the communication quality of the RoF networks. To improve the communication quality, the optical DSB signals need to be converted into optical single-sideband modulation (SSB) signals before fiber distribution in order to mitigate the dispersion-induced microwave power fading.
One method for the aforementioned optical DSB-to-SSB conversion uses optical filters to suppress one frequency component of the optical DSB signals. However, the optical power of the optical SSB signals is significantly reduced due to the power loss of the filtering process. Therefore, optical amplifiers are generally required to compensate for the power loss, which results in significant amount of unnecessary power loss and which increases the complexity of the device structure. In addition, since the central frequency and the pass-band of the optical filters are typically fixed, the conversion device cannot be reconfigured dynamically for different RoF networks adopting different operating microwave frequencies. Another method for the optical DSB-to-SSB conversion simultaneously applies microwaves with different phases to externally modulate semiconductor lasers. Since careful control of phase difference between the microwaves is required, this conversion device needs to be highly stable under possible ambience variation or operating condition adjustment. Therefore, the complexity of the structure and operation for this conversion device is high.
Consequently, how to improve the aforementioned conversion methods and devices or how to develop other conversion methods and devices with better device performance, simpler device structure, and/or easier device operation has become the focus of the people working in related areas and the emphasis of the present invention.